1. Field of the Invention
The invention relates to fiber-reinforced plastic panels, wood composites and to methods of manufacturing such panels, and in particular to fiber-reinforced plastic panels that are bonded using industry recognized wood adhesives.
2. Description of the Related Art
FRP-wood hybrids offer considerable potential for widespread use in construction and infrastructure applications. In addition to increasing the strength, stiffness and ductility of engineered wood composites, the hybrids allow for the utilization of low-grade lumber in construction. The hybrids also offer flexibility in design allowing for longer spans, lower depths, and lighter structures. One important factor in developing this hybrid technology is to provide adequate bond strength between the FRP reinforcement and the wood.
Pilot studies in the past five years by the inventors and others have shown the significant promise of combining wood and FRP. The inventor's studies have revealed, for example, that FRP reinforcement in the order of 2% can increase the bending strength of wood beams by over 50%.
The idea of reinforcing wood is not new. Many studies on wood reinforcement have been performed in the past 40 years. Often metallic reinforcement was used including steel bars, prestressed stranded cables, and stressed or unstressed bonded steel and aluminum plates. While significant increases in strength and stiffness have been achieved, the problems encountered were generally related to incompatibilities between the wood and the reinforcing material. Wood beams reinforced with bonded aluminum sheets experienced metal-wood bond delamination with changes in moisture content of only a few percent. The differences in hygro-expansion and stiffness between the wood and reinforcing materials can lead to separation at the glue-line, or tension failure in the wood near the glue-line.
To improve durability, fiberglass has been used in a number of ways, such as for beam reinforcement, as face material of wood-core sandwich panels, as external reinforcement for plywood, and in the form of prestressed strands. Unlike traditional steel and aluminum reinforcement, FRP reinforcement of wood composites can be successful because the physical/mechanical/chemical properties of the FRP are very versatile. The FRP may be engineered to match and complement the orthotropic properties of wood; consequently, incompatibility problems between the wood and the reinforcing FRP are minimized.
FRP materials (fibers/matrix) can be readily incorporated into many of the manufacturing processes currently used to produce structural wood composites.
In recent years, the bond strength of the FRP-wood interface has been investigated by a number of researchers and some related patents have been issued. U.S. Pat. No. 5,498,460, issued to Tingley, discloses a method of bonding FRP to wood using surface sanding to "hair up" fibers. In U.S. Pat. No. 5,362,545, also issued to Tingley, the inventor discloses a method to produce surface recesses on the FRP to enhance the bond strength.
In Forest Products Journal, Vol. 44, No. (5), pp. 62-66, 1994, Gardner et el. examined several different adhesive systems for use in glue-laminated wood structures. These included resorcinol formaldehyde (RF), an emulsion polymer isocyanate (EPI), and an epoxy resin. For composite joints fabricated from glass-reinforced vinylester or polyester FRPs and yellow poplar, the highest shear strengths were obtained using the RF adhesives, followed by the epoxy and EPI systems. The RF adhesive was also found to produce the greatest percentage of dry test failures within the wood matrix for wood-vinylester FRP matrix systems with values exceeding 90%. However, polyester-wood joints were found to produce almost 100% cohesive failures within the FRP matrix under dry conditions. Wet tests produced only 20 to 40% wood failures for vinylester-wood joints and 80% FRP matrix failures for polyester/wood joints.
Dailey et al. disclose in the Proceedings of Composites Institute's 50th Annual Conference & Expo '95, Composites Institute of the Society of the Plastics Industry, Inc., Cincinnati, Ohio, pp. 1-4, Session 5-C, the bonding of pultruded glass/phenolic composites to Douglas-fir. Both resorcinol formaldehyde (RF) and resorcinol-modified phenolic (PRF) adhesives were studied. Initial shear testing showed the PRF to outperform the RF. The authors varied such parameters as open and closed times, clamping pressure, and cure periods. They concluded that a long curing period increased the bond strength and emphasized the need for additional research to completely quantity the mechanical properties of the FRP-wood hybrids.
There is not found in the prior art a simple, inexpensive, commercially viable method of bonding FRP panels together or to other surfaces, such as wood, that uses common, proven wood adhesives, yet meets or exceeds shear bond strength requirements as specified under ANSI/AITC 190.1 and cyclic-delamination requirements under AITC 200.